Nigel G. Shrive

Stress governs tissue phenotype at the femoral insertion of the rabbit MCL

The cells in the midsubstance portion of skeletal ligaments typically have elongated shapes, but where ligaments insert into bone the cells appear very rounded and the tissue phenotype is that of fibrocartilage. Between the midsubstance and the insertions there is a gradient in cell shape and tissue phenotype that has been hypothesized to reflect a gradient of mechanical stresses. To test this hypothesis, cell shapes (an index of tissue phenotype) were quantified in the central part of the femoral insertion of the rabbit medial collateral ligament by computer-assisted histomorphometry. Morphometric measurements were correlated with the mechanical stresses and strains in the central part of the insertion as predicted by finite element analysis. Throughout the ligament the direction of the predicted principal tensile stresses coincides with the direction of the collagen fibers which curve from the midsubstance to meet the femur at nearly right angles. Principal compressive stresses also occur within the ligament: the highest are localized near the bone; the lowest in the midsubstance. The areas with the roundest cells correspond to the areas with the highest principal compressive stresses in the model; the areas with the flattest cells correspond to the areas with the lowest compressive stresses in the model. A correlation between cell shape and mechanical stresses suggests that physiological loading of the MCL is important for the maintenance of tissue phenotype throughout this insertion. We theorize that the cells in ligament insertions adapt to the prevailing local mechanical environment.

Wave intensity analysis and the development of the reservoir–wave approach

The parameters of wave intensity analysis are calculated from incremental changes in pressure and velocity. While it is clear that forward- and backward-traveling waves induce incremental changes in pressure, not all incremental changes in pressure are due to waves; changes in pressure may also be due to changes in the volume of a compliant structure. When the left ventricular ejects blood rapidly into the aorta, aortic pressure increases, in part, because of the increase in aortic volume: aortic inflow is momentarily greater than aortic outflow. Therefore, to properly quantify the effects of forward or backward waves on arterial pressure and velocity (flow), the component of the incremental change in arterial pressure that is due only to this increase in arterial volume—and not, fundamentally, due to waves—first must be excluded. This component is the pressure generated by the filling and emptying of the reservoir, Otto Frank’s Windkessel.

Altering ligament water content affects ligament pre-stress and creep behaviour

The water content of a ligament can be altered by injury and surgical intervention in vivo, and inadvertently or purposely during in vitro tests. We investigated how altering the water content of the rabbit medial collateral ligament (MCL) affected its resulting creep behaviour (defined as an increase in strain from sequential cyclic and static creep tests). The water content of normal MCLs (n = 4) was compared to that of MCLs soaked for 1 h in a sucrose solution (n = 4) or phosphate buffered saline (PBS; n = 8). Sucrose exposure decreased hydration and PBS exposure increased hydration. In addition, soaking in PBS caused a shift in ‘ligament zerO’ (the position where there was 0.1 N of tension on the ligament). Following the same single solution treatment, additional MCLs were creep tested at 4.1 MPa using a load based on the ligament cross‐sectional area measured before solution treatment: sucrose (n = 4), PBS new ‘ligament zerO’ (n = 5), and PBS old ‘ligament zerO’ (n = 6). Normal MCLs were also tested at 4.1 MPa (n = 7) in a humidity chamber that maintained normal ligament water content. Additional MCLs were treated with both solutions in series (n = 12) to examine the reversibility of the mechanical changes caused by single solution treatment. This was the first investigation to show that ligament creep behaviour was clearly affected by the initial state of hydration: creep decreased with decreased hydration and creep increased with increased hydration. Another unique finding was that ligaments with increased hydration had decreased ligament functional length and increased ligament pre‐stress. The creep behaviour of these ligaments was decreased if they were loaded from the pre‐stressed state compared to the unloaded state. These results suggest that maintenance of physiological water content is important for in vitro mechanical testing of ligaments and controlling the low‐load stress state of ligaments in situ.

Ligament creep recruits fibres at low stresses and can lead to modulus-reducing fibre damage at higher creep stresses: a study in rabbit medial collateral ligament model.

Ligaments are subjected to a range of loads during different activities in vivo, suggesting that they must resist creep at various stresses. Cyclic and static creep tests of rabbit medial collateral ligament were used as a model to examine creep over a range of stresses in the toe‐ and linear‐regions of the stress–strain curve: 4.1 MPa (n =7), 7.1 MPa (n = 6), 14 MPa (n = 9) and 28 MPa (n = 6). We quantified ligament creep behaviour to determine if, at low stresses, modulus would increase in a cyclic creep test and collagen fibres would be recruited in a static creep test. At higher creep stresses, a decrease in measured modulus was expected to be a potential marker of damage. The increase in modulus during cyclic creep and the increase in strain during static creep were similar between the three toe‐region stresses (4.1, 7.1, 14 MPa). However, at the linear‐region stress (28 MPa), both these parameters increased significantly compared to the increases at the three toe‐region stresses. A concurrent crimp analysis revealed that collagen fibres were recruited during creep, evidenced by decreased area of crimped fibres at the end of the static creep test. Interestingly, a predominance of straightened fibres was observed at the end of the 28 MPa creep test, suggesting a limited potential for fibre recruitment at higher, linear‐region stresses. An additional 28 MPa (n = 6) group had mechanically detectable discontinuities in their stress–strain curves during creep that were related to reductions in modulus and suggested fibre damage. These data support the concept that collagen fibre recruitment is a mechanism by which ligaments resist creep at low stresses. At a higher creep stress, which was still only about a third of the failure capacity, damage to some ligaments occurred and was marked by a sudden reduction in modulus. In the cyclic tests, with continued cycling, the modulus increased back to original values obtained before the discontinuity suggesting that other fibres were being recruited to bear load. These results have important implications for our understanding of how fibre recruitment and stress redistribution act in normal ligament to minimize creep and restore modulus after fibre damage.

Finite element analysis of the meniscus: the influence of geometry and material properties on its behaviour

A finite element model of the knee meniscus was developed to investigate the effects of various geometrical and material properties on the behaviour of the meniscus under compressive load. Factorial methods were used to determine the relative effect of varying the properties by +/-10% of their initial value. It was found that the stresses in the meniscus were more sensitive to geometry (meniscus width and radius of curvature of the femoral surface of the meniscus) than material properties. The model was also used to investigate the effect of incongruency between the radius of curvature of the femur and the femoral surface of the meniscus. It was shown that mismatch between the curvatures of the femur and meniscus has a large effect on the stresses both in the meniscus and in the underlying cartilage. The results from the study have implications for the design and development of meniscal repair devices and replacements.

Semitubular blade plate fixation in proximal humeral fractures: A biomechanical study in a cadaveric model

This study was undertaken to compare the fixation of the semitubular blade plate with that of the AO T plate. Cadaveric humeri (n=12 pairs) from an elderly population (41 to 89 years) had either a blade plate (n=12) or a T plate (n=12) fixed to them, subsequent to which a transverse osteotomy was performed. Mechanical testing in tension was performed in 1 series (n=5, blade plate; n=5, T plate) by applying a single maximal load to failure and in a second series (n=7, blade plate; n=7, T plate) by applying submaximal cyclic loading before failure was induced. Results showed that the fixation provided by the semitubular blade plate was significantly better (P < .05) than that of the T plate in those specimens subjected to submaximal cyclic loading before failure (series 2). This latter testing method contains some component of the clinical situation compared with monotonic distraction to failure. Based on these results plus favorable clinical results reported in the literature, the semitubular blade plate is possibly a better alternative to the T plate in the management of proximal humeral fractures that require open reduction and internal fixation.

A New Method of Measuring the Cross-Sectional Area of Connective Tissue Structures

There appears to be no generally accepted method of measuring in-situ the cross-sectional area of connective tissues, particularly small ones, before mechanical testing. An instrument has therefore been devised to measure the cross-sectional area of one such tissue, the rabbit medial collateral ligament, directly and nondestructively. However, the methodology is general and could be applied to other tissues with appropriate changes in detail. The concept employed in the instrument is to measure the thickness of the tissue as a function of position along the width of the tissue. The plot obtained of thickness versus width position is integrated to provide the cross-sectional area. This area is accurate to within 5 percent, depending mainly on alignment of the instrument and pre-load of the ligament. Results on the mid-substance of the rabbit medial collateral ligaments are repeatable and reproducible. Values of maximum width and thickness are less variable than those obtained with a vernier caliper. The measured area is considerably less than that estimated assuming rectangular cross-section and slightly less than that estimated on the assumption of elliptical cross-section.

Biomechanical behavior of scar tissue and uninjured skin in a porcine model

A new method to test axial and transverse tensile properties of skin was developed to improve our understanding of skin mechanical behavior, and how it changes following injury and formation of a scar. Skin tissue was evaluated at 70 days following full‐thickness wounding in juvenile female pigs (N=14). Samples were taken in the axial (cranial–caudal) and transverse (dorsal–ventral) directions, for both scar tissue and uninjured skin, and were evaluated mechanically in vitro using a protocol of stress relaxation followed by tensile failure. Uninjured skin was more compliant, with a larger toe‐in region, and faster load relaxation, in the axial direction than the transverse. Such directional differences were not present in high‐load responses, such as linear stiffness or failure properties. When compared with uninjured skin, scars displayed a similar linear stiffness, with considerably reduced failure properties, and reduced low‐load compliance. Scars showed no directional differences in low‐load behavior, viscous response, or failure properties. These findings suggest morphological changes that may occur with injury that are consistent with the viscoelastic and directional changes observed experimentally. This improved understanding of how injury affects skin biomechanical function provides valuable information necessary for the design of successful grafting procedures and tissue‐engineered skin replacements.

The Effects of Temperature on the Viscoelastic Properties of the Rabbit Medial Collateral Ligament

There are disparate views on the effects of temperature on the mechanical properties of ligaments and tendons. We attempted to resolve the inconsistencies by testing the medial collateral ligaments of twelve, three-month old New Zealand white rabbits in both elastic-dominated and viscous-dominated tests between 25 degrees C and 55 degrees C. We found that in elastic-dominated monotonic loading, the loading portions of the load-extension curves were mathematically similar. Differences could be accounted for through a base-line shift of the origin caused by additional relaxation and thermal contraction/expansion of the apparatus and specimen. In tests where the viscous component of behavior was manifest, we found results similar to those of other investigators. Thus we conclude that in assessing the effects of temperature on the mechanical properties of tissues it is important to account for both temperature and initial positions of the apparatus and specimen, and to consider the effects of both relaxation and thermal contraction/expansion.

Injury location affects ligament healing: A morphologic and mechanical study of the healing rabbit medial collateral ligament

Based on the heterogeneity of the rabbit medial collateral ligament (MCL) along its length, we tested the hypothesis that injury location would affect its healing response. The right MCL of 80 skeletally mature New Zealand white rabbits was sectioned adjacent to bone at the femoral end (40 rabbits) or the tibial end (40 rabbits) and reapposed with sutures. Animals were killed after 3, 6, 14, or 40 weeks of healing to examine wounds histologically (2 rabbits per healing interval) and mechanically (8 rabbits per healing interval). Results of the mechanical tests were compared to midsubstance MCL repairs (24 rabbits) and to uninjured normal MCLs (20 rabbits). The morphology of the near-insertion repairs was characterized by abnormal callus-like formation and patchy bone resorption, particularly at the tibial insertion. Mechanically, insertional injuries remodeled towards normal MCL low-load, viscoelastic and failure properties more slowly than midsubstance injuries at the early healing intervals. After 40 weeks of healing, few injury-specific differences persisted. All injured ligaments had ultimate strengths 15-35 percent short of normal at 40 weeks and the femorally-injured ligaments were weaker than normal at this time. These results suggest that rabbit MCLs, injured near either end, heal more slowly than those injured in their midsubstance and develop abnormal insertion morphology.